<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Document</title>
    <script src="jquery-3.3.1.min.js"></script>
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  <body>
    <div></div>
    <script>
      $("div").css("width", "300px");		// 设置width为300px
      console.log($("div").css("width"));	// 结果为：300px
    </script>
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</html>